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5. Pituitary
The pituitary gland, together with its connections to the hypothalamus, acts as the main endocrine interface between the central nervous system and the rest of the body. Location: base of skull in a concavity within the sphenoid bone called the sella turcica (pituitary fossa), immediately below the hypothalamus and optic chiasm. The pituitary is usually divided (in practice) into anterior and posterior parts, which actually refer to groupings of four subparts: * anterior pituitary (adenohypophysis) ** pars distalis ** pars tuberalis ** pars intermedia * posterior pituitary (neurohypophysis) ** pars posterior ** infundibular stalk ** median eminence The pituitary gland is covered superiorly by a dural reflection - the diaphragma sellae. This has an opening for the infundibular stalk. Anterior pituitary (adenohypophysis) The anterior pituitary is the largest part of the gland, and is responsible for synthesis and release of most pituitary hormones (with the exception of oxytocin and antidiuretic hormone (ADH) which are released by the posterior pituitary). It wraps anterolaterally around the posterior pituitary. Pars distalis The pars distalis is the largest part of the pituitary gland. It forms from the anterior wall of Rathke pouch. It is composed of cords of epithelial cells individually specialized to secrete tropic hormones acting on target organs: # growth hormone (somatotropin) # thyroid stimulating hormone (TSH) # adrenocorticotropic hormone (ACTH) # follicular stimulating hormone (FSH) # leutinizing hormone (LH) # prolactin Pars tuberalis The pars tuberalis is the part of the adenohypophysis which surrounds the anterior aspect of the infundibular stalk. Pars intermedia The pars intermedia in a thin layer of epithelial cells located between pars distalis and neurohypophysis. It arises from the posterior wall of Rathke pouch. It contains axons from hypothalamus and infundibulum. These may give rise to Rathke cleft cysts (also known as pars intermedia cysts). Posterior pituitary (neurohypophysis) The posterior pituitary (aka neurohypophysis) is a direct extension from the hypothalamus and does not synthesize any hormones, but rather releases oxytocin and ADH (vasopressin) which has travelled down the infundibulum in vesicles termed Herring bodies. The cell bodies reside into hypothalamic nuclei: * supraoptic nucleus * paraventricular nucleus The infundibular stalk is part of the neurohypophysis and extends from the tuber cinereum and pierces the diaphragma sellae before being surrounded by the pars tuberalis. Anteroposterior to it is the optic chiasma. Relations * inferiorly: basisphenoid, sphenoid sinus * superiorly: diaphragma sellae, suprasellar cistern * laterally cavernous sinus (and contents) * anteriorly: anterior intercavernous sinus * posteriorly: posterior intercavernous sinus Blood supply The pituitary has rich blood supply, with both a portal circulation (to the anterior pituitary) and arterial supply (to the posterior pituitary). Portal circulation The anterior pituitary receives blood which descends from the hypothalamus along the infundibulum as a portal system (venous channels connecting two capillary beds) * The superior hypophyseal arteries give superior branches which supply the superior most infundibulum that contain axons from a number of hypothalamic nuclei. * Hypophyseal portal venous plexus carries inhibiting/releasing factors from the axons in the infundibulum to the anterior pituitary where they control the release of its numerous hormones . Arterial circulation A rich network of small branches supply the stalk and the posterior pituitary, with a total of six arteries described, three from above and three from below/side (from the cavernous segment internal carotid artery). These arteries anastomose with each other. * from above ** superior hypophyseal artery (from the internal carotid artery) ** infundibular artery (from the posterior communicating artery) ** prechiasmal artery (from the ophthalmic artery) * from below/side (cavernous carotid artery) ** inferior hypophyseal artery (from the meningohypophyseal trunk) ** capsular artery ** artery of the inferior cavernous sinus Venous system Petrosal sinuses into the cavernus sinuses Variant anatomy The main variations are in size/number: * hypoplasia * hyperplasia: physiologic prolactin cell hyperplasia due to lactation or pregnancy and in response to end organ failure such as Addison disease, Klinefelter or Turner syndrome, or primary hypothyroidism * partially empty sella turcica (see: empty sella sign) - diaphragma sellae is incomplete and CSF may enter into the pituitary fossa and give rise to the "empty sella sign" on imaging. The pia and arachnoid blend with the capsule of the gland. * duplication